Vacuum tubes are arrangements of electrodes in an envelope of glass or other material within which a vacuum has been applied. The electrodes are attached to leads passing through the envelope for connection to an electrical circuit. Simple vacuum tubes comprise a filament sealed in a glass envelope evacuated of all air. Upon heating of the filament by a process called thermionic emission, electrons are released form the filament. The released electrons, bearing a negative electrical charge, will move through the vacuum towards a positively-charged metal plate anode, resulting in a flow of electrons from the filament to the anode.
Vacuum tubes have been used for a wide variety of electronics applications, and are still used for specialised audio amplifiers. Vacuum tube principles are used in cathode ray tubes in televisions, oscilloscopes and computer monitors.
It is known to provide a device for x-ray imaging. An example of such a device includes U.S. patent application Ser. No. 10/795,414 by Kabushiki Kaisha Toshiba (“Toshiba”). The Toshiba patent describes an x-ray image tube device having a vacuum tube enclosing a main body. The main body has a photoelectric surface for converting light into electrons. Focusing electrodes along the length of the vacuum tube act as an electron accelerator and electron focuser directing electrons to an anode (electron collector) at the other end of the vacuum tube.
There are a number of desirable objectives in relation to electron emitters. Such electron emitters should ideally be able to operate without heating. They should be operable using a variety of electron activators, including various light sources. Existing electron emitters achieve some of these objectives, but with varying degrees of success.